In recent years, as high recording capacity and high density recording have been demanded, it has been strongly desired for magnetic recording media to have improved recording density.
As the recording medium for high density recording, the so-called metal thin film type magnetic recording medium having a ferromagnetic metal thin film formed by to the vacuum film forming methods such as the vacuum vapor deposition method or the sputtering method on a non-magnetic support is suitable, and its practical application has been promoted.
That is, the thin film type magnetic recording medium has high coercive force and squareness ratio, as well as excellent electromagnetic characteristics in short wavelength recording, and also is thin in thickness of the magnetic layer, thus having excellent characteristics as a magnetic recording medium for high density recording, such as low recording demagnetization or thickness loss during reproduction.
And, among them, a metal thin film type magnetic recording medium using a Co--Ni type alloy for the magnetic layer has good magnetic characteristics. Particularly, an alloy comprising about 80 atomic % of Co and 20 atomic % of Ni can provide a metal thin film type magnetic recording medium which is low in deterioration of magnetic characteristics and excellent in weathering resistance.
However, the metal thin film type magnetic recording medium using a Co--Ni type alloy as the magnetic layer has the problem that S/N could not be increased as expected due to great noise.
As the means for reducing noise, there has been an attempt to make crystal grains constituting the magnetic layer finer by vapor depositing a Co--Ni type alloy on a non-magnetic support, and various contrivances about how to introduce oxygen gas have been proposed.
For example, JP-A-58-41442 and JP-A-58-83328 (the term "JP-A" as used herein means an "unexamined published Japanese patent application) disclose methods of introducing oxygen from the higher incident angle side in vapor depositing obliquely a Co--Ni type alloy.
JP-A-58-41443 and JP-A-58-83327 disclose methods of introducing oxygen conversely from the lower incident angle side.
Also, methods of introducing oxygen gas with concern for the incident angle of a vapor stream of a Co--Ni type alloy are disclosed in JP-A-62-102427, JP-A-60-157728, JP-A-62-26639, and JP-A-62-121929.
JP-A-60-154323 discloses a magnetic layer with particle size of a Co--Ni type alloy of 50 to 100 .ANG., and particle size of a Co--Ni oxide of 30 to 70 .ANG. according to analysis by TEM (transmission type electron microscope) by introducing oxygen gas from the minimum incident angle.
However, even according to the methods proposed in the prior art, reduction of noise cannot be said to be satisfactory, but Y-output is lowered, consequently failing to make S/N greater. Furthermore, the size of crystal grain did not correspond to S/N.
The present inventors have investigated with a presumption that the crystallite size of the Co--Ni type alloy may be more important than the size of crystal grains in considering the noise of the magnetic layer of the Co--Ni type alloy. Consequently, they found that the crystallite size as determined by the thin film X-ray diffraction method and noise directly correlate.
Also, it has been found that, according to the film-forming methods of the prior art as mentioned above, only films with the crystallite size in the magnetic layer of the Co--Ni type alloy of 50 .ANG. or more as seen in the (100) face of .alpha.-Co, or 130 .ANG. or more as seen in (002) face of .alpha.-Co could be obtained.